It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added between the selective reduction catalyst and the particulate filter, thereby attaining lessening of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between a urea water added position and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, a compact exhaust emission control device as shown in FIGS. 1 and 2 has been proposed by the inventor as Japanese patent application No. 2007-29923. In the exhaust emission control device of the prior application shown, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particles in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8 is a selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. A discharge end of the particulate filter 5 is connected to an entry end of the selective reduction catalyst 6 through an S-shaped communication passage 9 such that the exhaust gas 3 discharged through the discharge end of the particulate filter 5 is reversely curved back into the entry end of the adjacent selective reduction catalyst 6.
As shown in FIG. 2 which shows substantial parts in enlarged scale, the communication passage 9 is the S-shaped structure comprising a gas gathering chamber 9A which encircles the discharge end of the particulate filter 5 to gather the exhaust gas 3 just discharged from the discharge end of the particulate filter 5 through substantially perpendicular turnabout of the gas, a mixing pipe 9B which extracts the gathered exhaust gas 3 from the chamber 9A in a direction reverse to the flow of the exhaust in the particulate filter 5 and which is provided with a urea water addition injector 11 (urea water addition means) intermediately of the mixing pipe and a gas dispersing chamber 9C which encircles the entry end of the selective reduction catalyst 6 so as to disperse the gas 3 guided by the mixing pipe 9B through substantially perpendicular turnabout of the gas into the entry end of the selective reduction catalyst 6.
Arranged in the casing 7 and in front of the particulate filter 5 is an oxidation catalyst 14 for oxidization treatment of unburned fuel in the exhaust gas 3, and arranged in the casing 8 and behind the selective reduction catalyst 6 is an ammonia lessening catalyst 15 for oxidization treatment of surplus ammonia.
With such construction being employed, particulates in the exhaust gas 3 are captured by the particulate filter 5. The urea water is added intermediately of the mixing pipe 9B and downstream of the filter into the exhaust gas 3 by the injector 11 and is pyrolyzed into ammonia and carbon dioxide gas, so that NOx in the exhaust gas 3 is favorably reduced and departed by the ammonia on the selective reduction catalyst 6. As a result, both the particulates and NOx in the exhaust gas 3 are lessened.
In this case, the exhaust gas 3 discharged through the discharge end of the particulate filter 5 is reversely curved back by the communication passage 9 into the entry end of the adjacent selective reduction catalyst 6. As a result, enough reaction time is ensured for production of ammonia from the urea water since a long distance between the urea water added position intermediately of the communication passage 9 and the selective reduction catalyst 6 is ensured and the flow of the exhaust gas 3 becomes turbulent due to the reversed curving to facilitate mixing of the urea water with the exhaust gas 3.
Moreover, the particulate filter 5 and selective reduction catalyst 6 are arranged in parallel with each other and the communication passage 9 is arranged between and along the particulate filter 5 and selective reduction catalyst 6, so that the whole structure becomes compact in size to substantially enhance its mountability on a vehicle.
Where the addition of the urea water to the selective reduction catalyst 6 is to conducted between the particulate filter 5 and the catalyst 6 as illustrated in FIGS. 1 and 2 and mentioned in the above, the mixing pipe 9B is provided midway with a boss 10 branching slantingly to upstream side. Fitted from outside of the mixing pipe 9B into the boss 10 is a urea water addition injector 11. With the injector 11 protected so as not to be directly exposed to the flow of the hot exhaust gas 3, urea water is added so that, where the exhaust gas 3 has increased flow rate as shown in FIG. 3, the urea water added by the injector 11 is forced by the vigorous flow of the exhaust gas 3 to be biased along an inner wall of the mixing pipe 9B, disadvantageously failing in sufficient dispersion of the urea water.
Then, the inventor devised out as shown in FIGS. 4 and 5 that a side surface of the entry end of the mixing pipe 9B adjacent to the discharge end of the particulate filter 5 is formed with an opening 12. Gas guide passages 13 for introduction of all of the exhaust gas 3 from the discharge end of the particulate filter 5 tangentially into the opening 12 are formed, using guide fins 16a and 16b and a part of the discharge end of the gas gathering chamber 9A. A urea water addition injector 11 is coaxially fitted to an entry end face of the mixing pipe 9B so as to add the urea water axially of the entry end of the mixing pipe 9B.
Specifically, in this manner, the exhaust gas 3 tangentially flows into the opening 12 to make a spiral flow in the mixing pipe 9B, which facilitates dispersion of the urea water axially added to the entry end face of the mixing pipe 9B by the urea water addition injector 11 and facilitates collision of the urea water against an inner periphery of the mixing pipe 9B; as a result, mist particles of the urea water are effectively miniaturized and early decomposed into ammonia and carbon dioxide gas.
As a prior art literature pertinent to the invention, there already exists, for example, the following Patent Literature 1.    [Patent Literature 1] JP 2005-155404A